Inkjet printers operate a plurality of inkjets in each printhead to eject liquid ink onto an image receiving member. The ink can be stored in reservoirs that are located within cartridges installed in the printer. Such ink can be aqueous ink, UV Curable, ink emulsions, or phase-change inks. Phase-change inkjet printers receive ink in a solid form and then melt the solid ink to produce liquid ink for ejection onto the imaging member. The printer supplies ink to printheads for ejection through inkjets onto an image receiving member of an image receiving member, such as a print medium or an indirect imaging belt or imaging drum. Liquid inks dry or are cured and phase change inks cool into a solid state after being transferred to a print medium, such as paper or any other suitable medium for printing.
In an inkjet printer, an ejected ink drop travels through air for a predetermined distance before reaching an image receiving member. Examples of image receiving members include paper or other print media, or indirect imaging members such as imaging drums or endless belts that receive ink images for later transfer to a print medium. Printers are generally configured to minimize the “time of flight”, which is the time delay between ejection of an ink drop and arrival of the ink drop on image receiving member. Longer time of flight delays tend to result in poorer placemen of ink drops and may result in a degradation of printed image quality. In an inkjet printer, the time of flight for ejected ink drops is influenced by both the distance between the printhead and the image receiving member and the velocity profile of the ink drop as the ink drop moves from the inkjet to the image receiving member. The velocity of the ink drop tends to decrease after ejection due to the presence of drag from the air around the ink drop. Thus, as the distance between the printhead and the image receiving member increases, the flight time increases both due to the increased distance and due to the reduction in ink drop velocity due to drag. Additionally, inkjet printheads eject ink drops at a maximum velocity that is related to the volume of the ink drop. In general, the printheads eject ink drops with larger volumes at higher initial velocities than ink drops with smaller volumes.
Given the limitations cited above, many printers place the printheads in close proximity to the image receiving member with gaps of, for example, one millimeter or less between the inkjets in the printhead and the image receiving member. Additionally, the printers often eject ink drops with greater volumes to minimize the time of flight for the ink drops. The close proximity of printheads to an image receiving member can result in damage or contamination of the printheads, particularly when the image receiving member is paper or another print medium that may make contact with the printhead. Additionally, some printed images are reproduced with higher quality when the printhead is configured to eject smaller ink drops. The drop placement errors that result from the increased time of flight for the smaller ink drops may preclude the use of smaller ink drop sizes during an imaging operation, however.
One approach to reducing ink drop time of flight that is known to the art uses an electrostatic field (E-field) to accelerate an ink drop from the printhead face to the image receiving member. The printhead ejects the ink drop using an ordinary actuator, such as a piezoelectric transducer or a thermal actuator, but the ink drop receives a static electric charge as the ink drop leaves the printhead. The electrostatic field generated between the printhead and the image receiving member produces a charge with an opposite polarity on the image receiving member, and the attraction between the charged ink drop and the image receiving member accelerates the ink drop and reduces the time of flight in comparison to ejecting the ink drop in the absence of the electrostatic field.
While electrostatic fields are known to the art, the electrostatic fields also affect in-flight ink drops in ways that tend to contaminate printheads that effectively prevent the use of electrostatic fields in widely deployed inkjet printers. As is known in the art, inkjets eject some ink drops in a manner where a larger ink drop is accompanied by one or more smaller “satellite” ink drops. In some instances, a satellite ink drop is formed from a larger ink drop after the larger ink drop is ejected from an inkjet. If the satellite ink drops land on the image receiving member in close proximity to the larger ink drop, then the quality of the printed image is preserved. However, in a printer that uses an electrostatic field, some of the satellite ink drops are formed with the charge polarity of the image receiving member instead of the printhead. For example, if the electrostatic field forms a positive charge on the ink drops and a negative charge on the image receiving member, some of the satellite ink drops receive a negative charge. The negative charge repels the satellite ink drops from the image receiving member and attracts the satellite ink drops back to the printhead, where the ink contaminates the printhead and results in clogged inkjets. Over time, the ink contamination due to the electrostatic field degrades the quality of printed images and increases the requirements for cleaning and maintenance of the printheads. In light of these deficiencies, improvements to printers that enable printing ink drops with reduced time of flight while reducing or eliminating printhead contamination would be beneficial.